The present invention relates generally to the field of apparatus and methods for gathering and analyzing samples of gas, and, more particularly, to systems for automatically collecting a plurality of air samples for subsequent analysis at a remote location.
There are many applications in which it is desirable to collect gas samples from a particular environment, and to subsequently analyze these samples for certain constituents, and in particular for minor or trace constituents. A typical application for this technology is the United States manned space flight program where archival air samples are collected from the interior of a spacecraft, and subsequently analyzed for constituents of the samples at a ground based location. Analysis of these samples has been a key component of the assessment of spacecraft air quality for crew health purposes. Periodic analysis at predetermined time intervals, such as intervals during a flight, is also important in the overall air quality assessment program.
In using air sampling techniques in ground-based applications, sampling is typically linked to eight-hour time intervals to correctly assess an employee""s exposure during a typical xe2x80x9cwork dayxe2x80x9d. In using air-sampling techniques in space flight applications, the xe2x80x9cwork dayxe2x80x9d of a spacecraft crew is typically twenty-four hours. Although experience has shown that spacecraft air quality regarding major constituents such as oxygen and nitrogen is stable over long periods such as months, substantial changes in trace constituents or contaminants can occur over a twenty four hour period as the crew engages in different activities such as exercise and experiments. Therefore, a twenty-four hour integrated sample of spacecraft air would most accurately represent crew exposure to possible air contaminants.
Under normal circumstances, contaminant concentrations during shorter sampling periods might be unduly biased by one or more crew activities during the sample time, and longer sampling time periods would tend to average or xe2x80x9cflattenxe2x80x9d contaminant fluctuations. A twenty-four hour sample collects a sufficient quantity of certain constituents that are present in very low concentrations so that such constituents might not otherwise be detected. Furthermore, automatic sampling is desirable. Conditions can be encountered, however, when shorter sampling periods are desirable, and when manual initiation of the sampling is required.
In effectively sampling air for human use, means are required which trap volatile organic contaminants, but not major constituents such as nitrogen, oxygen, carbon dioxide, and water. In space flight applications, sampling means which conserve power are also desirable since spacecraft power is limited. Prior art systems employ a solid sorbent air sampler unit (SSAS) to monitor spacecraft cabin air. The SSAS consists of a plurality of tubes containing a sorbent and into which sampled air is directed by mean of a valving system. The SSAS is a manually activated device that requires a crew member to manipulate a valve to collect an air sample in a tube containing sorbent material and then record the time of this sample session.
Manual operation is subject to human error or mechanical and has resulted in samples which are unusable. For example, batteries which operate an SSAS system may be exhausted during a mission, resulting in unusable samples. The sampling acquisition valve may not be switched to the next sample tube in a timely manner resulting in an unusable sample due to a sample xe2x80x9coverloadxe2x80x9d of the sorbent in the tube. Sample collection times may go unrecorded, or be recorded improperly, or recorded illegibly, once again resulting in an unusable sample. The SSAS sample pump used to draw samples into the sample tube may malfunction, but notice of the malfunction may not be provided, resulting in questionable samples in all tubes rendering the samples unusable.
In addition to the sampling problems and lack of automation discussed above, various flow rate orifices are subject to clogging. Changing tube sorbents is extremely difficult because sample tubes must be removed from the sampling device. The overall configuration of the system is not ideally suited for this procedure.
Other air sampling systems have been used on board spacecraft. These systems, like the SSAS, suffer from certain shortcomings. For example, glass-lined sample collection tubes present a breakage hazard. Such tubes require disassembly after sampling so that they can be cleaned in order not to contaminate subsequent sampling cycles. Such systems may also suffer cross contamination of samples due to multi-port valve design, and o-ring contamination of drawn sample gases.
In view of these prior art methods and apparatus, an object of the present invention is to provide a gas sampling system which can be used to collect a number of samples of gas for subsequent analysis.
Another object of the invention is to provide a system which employs sorbents in sample tubes, and the sorbents are sensitive to trace contaminants but not the major constituents of air.
Yet another object of the present invention is to provide sampling tubes which can be adjusted in size to accommodate varying amounts of sorbent, which are silica-lined metal and therefore have inert surfaces, yet do not present a breakage hazard, and from which samples can be drawn for analysis without physically removing the sample tube from the sample apparatus.
Another object of the invention is to provide a fully programmable sampling sequence which can draw samples at predetermined times and for predetermined intervals, with manual override capabilities which can be used to draw a sample at the discretion of an operator.
Still another object of the invention is to provide a system which can be cleaned, perform sampling, and then be desorbed without disassembling the device. This insures sample integrity and prevents any contamination from the analytical processes.
Another object of the present invention is to provide a sampling system which uses minimal electrical power thereby extending the life of batteries operating the system for long sampling periods.
Although the invention is directed toward sampling air in spacecraft, it should be understood that the invention can be used for other applications such as sampling air in the vicinity of hazardous materials manufacturing facilities, sampling air in contained environments, and the like. Other objects and applications of the invention will become apparent in the following disclosure.
The sampling system of the present invention will be referred herein to as an Automated Multi-Sorbent Tube Air Sample, or xe2x80x9cAMTASxe2x80x9d. The apparatus comprises a xe2x80x9cmulti-portxe2x80x9d valve and 16 sorbent tubes. Each tube is connected to the multi-port valve by means of pairs of the valve ports. The multi-port valve is constructed and operated in a predetermined sequence to create a flow path through the multi-port valve. The flow path directs an air sample to only one connected sorbent tube at a given time and for a given time interval.
In operation, a gas sample flows from a sample source, such as air from a spacecraft cabin, through an inlet filter and intake line, into a primary inlet port of the multi-port valve, through the multi-port valve, through the sequentially connected sorbent tube, back through the multi-port valve and out through a primary outlet port, through a pump, and then through a pump exhaust. The action of the pump draws the sample into and through the connected sorbent tube.
The sequencing of the multi-port valve, the time at which flow is initiated through a given sorbent tube, and the duration of the flow through a given sorbent tube, is controlled by a processor such as a computer. The movement of the multi-port valve is controlled by an actuator cooperating with the computer, which moves the valve to one of thirty two positions, for example, under the control of the computer. Sixteen of the valve positions discretely align a given sorbent flow tube in the sample collection geometry as described above. The other sixteen valve positions are xe2x80x9cparkxe2x80x9d positions as will be described below.
The AMTAS is capable of collecting sixteen archive samples, and the computer is preprogrammed to collect the samples at predetermined times and for predetermined intervals. The preprogrammed collection times and intervals can, however, be manually overridden by an operator such as a spacecraft crew member to obtain an xe2x80x9cunscheduledxe2x80x9d sample. The entire system is battery powered.
The sorbent tubes are constructed of silica-lined metal tubing and in multiple loops so that the volume of the tube can be adjusted to accommodate varying amounts of sorbent to capture particular contaminants of interest. The pump, which draws samples into and through the sorbent tubes, is operated in a pulsed mode only during sampling thereby conserving battery power. After a preset sampling time, the computer processor either switches the multi-port valve to the next sorbent tube, or moves the valve to one of the sixteen park positions.
In a park position, the valve is set between ports, thereby isolating all sorbent tubes in this position. This saves energy since the multi-port valve then needs to be turned only a fraction of an inch to align the intake and exhaust ports of the next sorbent tube when called for by the control processor. This prevents the power drain of the valve actuator having to rotate repeatedly to a single designated parking position between sampling intervals, and then returning to the next sorbent tube called for by the computer controlled sample sequence.
During the collection of samples, the AMTAS cooperates with a clock and an output device to automatically record the time of initiation and the duration of each sampling. The system also logs all other sampling operations including error messages should any system malfunction occur.
The system can also monitor any unusual event. For example, one sorbent tube may be reserved for contingency purposes. Should such a previously identified contingent event occur, an operator may slide a mode switch from AUTO to MANUAL and depress a guarded button, thereby automatically causing the control processor to rotate the multi-port valve to the specified tube and the sample collection pump to run for a predetermined time period.
Once the sampling system is returned to a facility for analysis of the collected samples, the pump is disconnected from the multi-port valve at the primary outlet port, and a source of ultra pure nitrogen is connected to this port. Each sorbent tube, as it is selected for desorption, is released from a clamp and bent downward approximately ninety degrees by means of coil spring fittings and placed into a desorption heater without removing the tube from the AMTAS. Other tubes are unaffected during this process. A clean, proofed, evacuated gas sample container (GSC) is attached to the sample inlet fitting, nitrogen is directed through the multi-port valve and through the sorbent tube being heated, back through the multi-port valve and inlet fitting and into the GSC. The sorbent tube is then removed from the heater and cooled. The multi-port valve is closed to isolate the GSC, and the GSC is removed from the system for analysis with a gas chromatograph/mass spectrometer.
After removing the sorbent sample using the carrier gas method, each sorbent tube is heated with the nitrogen flow to purify the sorbent, ready for the next sampling sequence. It is unnecessary, therefore, to disassembly to AMTAS for desorption and cleaning prior to repeated usage. The sample tubes are mounted in the system on coiled tubing xe2x80x9cspringsxe2x80x9d, which permit the sample tubes to be individually selected for heating/desorption, while the tubes remain connected into the system, and all other sample tubes remain undisturbed until selected.
Thus, in summary, it is a feature of the present invention to provide an automated sampling system in which a plurality of samples of trace elements present in ambient air are gathered under the control of a processor. It is a further feature of this invention to stage a plurality of sample tubes in minimum of space. It is a still further feature of the invention to position inlet and outlet ports of the sample tubes in adjacent ports of a multi-port valve, resulting in a looped-tube design. It is yet another feature of the invention to provide a xe2x80x9cparkxe2x80x9d position in the automated sampling system so that an energy-saving minimum of travel is required to move the sampling system from a sample position to a position in which all sample tubes are isolated. It is still another feature of the invention to mount the sample tubes on coiled tubing springs so that a sample can be desorbed and analyzed while the sample tube remains coupled into the sampling system. Finally, it is yet another feature of the invention to permits cleaning, sampling, and desorption of the sample tubes without removing the sample tubes from the system, while still permitting easy replacement of one or more tubes as required.
These and other features and advantages of this invention will be readily apparent to those skilled in the art.